Service Agreements are tied to the Contacts' End User Company that they are created from. In most cases the End User will also be the Bill To Customer, but sometimes it may be different. If the Bill To is different, you can drag and drop a Contact or Company Location from Search e-manage, Recent Records, Favorites or any grid that contains contacts or company locations onto the Bill To Label in the resulting window.
Service Agreements can hold many serviceable items from different locations to bill to and perform preventative maintenance on. A service agreement without any serviceable items will not allow you to invoice, because the sum of the serviceable items' price is the price of the entire agreement. If you are creating a Service Agreement from this menu item, you will either need to have serviceable items already set up in e-manage|ONE or you can create new ones from the Service Agreement Window after creating it.
Click the New Service Agreement action menu item